1. Field of the Invention
The present invention relates to a method of coding video signals, and more particularly, to a video signal coding method by which digital video signals are transformed into highly-efficiently-coded data which can be recorded with enhancement of the picture quality by a disk recorder.
2. Description of the Prior Art
A video signal recording system has been proposed in which intra- and inter-frame coded data obtained by highly-efficient-coding of video signals representing motion pictures are recorded on a recording medium, such as, a CD (compact disc), so as to be readily searched.
Highly-efficient coding is attained as follows:
By way of example, as illustrated in FIG. 1(A), motion pictures PC1, PC2, PC3, . . . are digitally-coded at times t=t1, t2, t3, . . . When being transmitted to a transmission system, for example, constituted by a CD recording system, transmission efficiency is enhanced by compressing the digital data to be transmitted while making use of the substantial autocorrelativity of the video signals.
More specifically, in effecting an intra-frame coding process in respect to the pictures PC1, PC2, PC3, . . . , arithmetic processing is performed to obtain the difference between one-dimensionally or two-dimensionally adjacent picture data along, for instance, a horizontal scanning line. Subsequently, the compressed bit-number picture data of the respective pictures PC1, PC2, PC3 . . . are transmitted.
For carrying out an inter-frame coding process, picture data PC12, PC23, . . . , for example, as shown in FIG. 1(B), and which consist of differences in pixel data between the adjacent pictures PC1, PC2 and between the adjacent pictures PC2, PC3, . . . , respectively, are sequentially obtained. The resulting picture data are transmitted, together with the intra-frame-coded picture data corresponding to the initial picture PC1, at the timing t=t.sub.1.
Thus, it is possible to supply, to the transmission system, video signals which have been highly-efficiently-coded so as to obtain digital data having a remarkably smaller number of bits than would be required for transmission of all of the pixel data of the pictures PC1, PC2, PC3, . . .
The above previously proposed video signal coding process may be executed by a picture data generating device 1 constructed as shown in FIG. 2, in which the incoming video signal VD is quantized to highly-efficiently-coded data D.sub.VD in a video signal coding circuit 2. The data D.sub.VD is temporarily stored in a transmission buffer memory 3 and is read therefrom as transmission data D.sub.TRANS at a predetermined transmitting velocity. The transmission data D.sub.TRANS is transmitted through a transmission path or route 4 to a picture data recording/reproducing device 5 which may be, for example, a CD recording/reproducing device. The transmission buffer memory 3 transmits the transmission data D.sub.TRANS at a transmitting velocity determined by the transmission capacity of the transmission path 4 leading to the picture data recording/reproducing device 5. Simultaneously, the transmission buffer memory 3 feeds back a remaining quantity data signal D.sub.RM through a feedback loop 6 to the video signal coding circuit 2. Such signal D.sub.RM indicates the quantity of data remaining in the memory 3. As a result of such feedback, the video signal coding circuit 2 controls the quantity of the highly-efficiently-coded data D.sub.VD supplied to the transmission buffer memory 3 by controlling a quantization step STEPG (FIG. 3) employed in digitally-coding the video signal VD. The data held in the memory 3 are thereby controlled so as to avoid an overflow or underflow.
The video signal coding circuit 2 of the known picture generating device 1 may, as shown specifically in FIG. 4, include a preprocessor 11 which receives the video signals VD and transforms a luminance signal and a chroma signal contained therein into digital data. Then, the preprocessor 11 executes a one-side field removing process and a one-side field line cull-out process so as to transform the digital data into motion picture data. The motion picture data is then transformed into transmission unit block data S11 with each block consisting of 16 pixels in the horizontal or line direction.times.16 lines of data. The resulting transformed data S11 are accumulated in a present frame memory 12 to provide present frame data S12 which is supplied to a subtractor circuit 13 as an addition input. Preframe data S13 obtained from a preframe memory 14 are also applied to the subtractor circuit 13 for subtraction in the latter from the present frame data S12. Thus, the difference or deviation data S14 obtained at an output terminal of the subtractor circuit 13 corresponds to the deviation between the transmission unit block data of the present frame picture data and the transmission unit block data of the preframe picture data. Such deviation data S14 is transformed into transform coding data S15 by means of a transform coding circuit 15 which, for example, may be constituted by a discrete cosine transform circuit. The data S15 is thereafter quantized in a quantization circuit 16.
Quantization data S16 obtained from the quantization circuit 16 is highly-efficiently-coded once again in a variable-length coding circuit 17. The resulting variable length coding data S17 is composited with pieces of first and second management information S18 and S19 in a composition circuit 18. The composite data is supplied, as transmission picture data S20, from the composition circuit 18 to the transmission buffer memory 3.
Additionally, the quantization data S16 is inverse-transformed by means of an inverse transform circuit 19 which includes an inverse quantization circuit and an inverse transform coding circuit (not shown). The inverse-transformed data S21 are accumulated as decoding deviation data in the preframe memory 14 via an adder circuit 20. The present frame picture data sent to the transmission buffer memory 3 are accumulated, as the preframe picture data, in the preframe memory 14.
On the other hand, a motion compensating circuit 21 is supplied with the present frame data S12 from the present frame memory 12 together with preframe data S22 from the preframe memory 14. Motion vector data S23 is formed by circuit 23 for indicating motion appearing from the preframe picture data in respect to the present frame picture data. The motion vector data S23 is supplied to the preframe memory 14 and is also supplied, as the first management information S18, to the composition circuit 18. As a consequence of the foregoing, the motion vector data S23 is transmitted to the transmission buffer memory 3 as part of the header information of the data corresponding to the deviation data S14.
The variable-length coding circuit 17 is supplied with quantization step data S24, as a control signal for the circuit 17 which represents the size of the quantization step STEP G employed for quantization by the quantization circuit 16. The quantization step data S24 is also supplied as the second management information S19 to the composition circuit 18. This information is composited in the transmission picture data S20 as a part of the header information of the deviation data S14.
With the above-described arrangement of the video signal coding circuit 2, when transmitting the picture data PC1 of FIG. 1(A) at the time t.sub.1 in the form of intra-frame-coded data, data of a value [0], that is representing the absence of picture, is provided as the preframe data S13 supplied to the subtractor circuit 13. Therefore, the present frame data S12 is supplied, as deviation data S14, directly to the transform coding circuit 15 via the subtractor circuit 13.
At this time, the transform coding circuit 15 transmits to the quantization circuit 16 transform coding data S15 which has been intra-frame-coded. The intra-frame-coded data is thereby transmitted as part of the transmission picture data S20 to the transmission buffer memory 3. Simultaneously, the relevant deviation data S14, that is, the present frame data S12 at such time, is decoded as decoding deviation data S21 by the inverse transform circuit 19 and accumulated in the preframe memory 14.
After the picture data PC1 has been transmitted as the intra-frame-coded data, that is, during the time t.sub.2, the picture data PC2 is supplied as the present frame data S12 to the subtractor circuit 13, and, at that time, the picture data PC1 is supplied from the preframe memory 14, as the preframe picture data to the subtractor circuit 13. As a result, the subtractor circuit 13 obtains deviation data S14 corresponding to the picture data PC12 of FIG. 1(B) representing the deviation between the picture data PC2 serving as the present frame data S12 and the picture data PC1 serving as the preframe data S13.
The deviation data S14 is transmitted to the transmission buffer memory 3 through the transform coding circuit 15, the quantization circuit 16, the variable-length coding circuit 17 and the composition circuit 18 so as to be included in the transmission picture data S20. The transmission picture data S20 is decoded in the inverse transform circuit 19 and then supplied, as the decoding deviation data S21, to the adder circuit 20.
At this time, that is, at the time t.sub.2, the adder circuit 20 adds the decoding deviation data S21 to the preframe data S13 for representing movement of a picture held in the preframe memory, such as, the picture data PC1, into a position shifted according to the motion vector data obtained from the motion detecting circuit 21. The present frame picture data is predicated on the basis of the preframe data and is then held in the preframe memory 14.
Transmitted to the motion detecting circuit 21 at this moment are picture data PC1 constituting the preframe picture data held in the preframe memory 14, and the motion vector data S23 then provided by the circuit 21 expresses a motion of the picture data which has come to the circuit 21 from the memory 12 as the present frame data S12. The result of adding the decoding deviation data S21 and the preframe picture data S13 is stored, as a vector position expressed by the motion vector data S23, in the preframe memory 14. The motion vector data S23 is simultaneously transmitted as part of the transmission picture data S20 through the composition circuit 18.
In the video signal coding circuit 2, when transmitting the picture data PC2 of FIG. 1(A) at the time t.sub.2 for obtaining inter-frame-coded data, the picture data PC12, representing a deviation between the preframe picture data PC1 and the present frame picture data PC2, is highly-efficiently-coded into inter-frame-coded data including the deviation data S14 and the motion vector data S23. Such inter-frame-coded data is supplied to the transmission buffer memory 3.
Similarly, at the times t.sub.3, t.sub.4, . . . , when new picture data is provided from the memory 12 as the present frame data S12, the present frame data S12 is highly-efficiently-coded into inter-frame-coded data by employing the preframe picture data S13 held in the preframe memory 14. Such highly-efficiently-coded data is then transmitted to the transmission buffer memory 3.
The transmission picture data S20 received in this manner by the memory 3 and temporarily stored therein are sequentially read from the memory 3 as transmission data D.sub.TRANS, at a predetermined data transmitting velocity determined by the transmission capacity of the transmission path 4 to the picture data recording/reproducing device 5 (FIG. 2). Remaining quantity data S25, representing the quantity of data remaining in the memory 3, is fed back to the quantization circuit 16 as a quantization size control signal, thereby controlling the quantity of data generated and supplied as transmission picture data S20 from the video signal coding circuit 2.
When the quantity of data remaining in the transmission buffer memory 3 increases up to an allowable upper limit, and the situation causing such increase is allowed to remain unchanged an overflow will eventually be induced, that is, the data quantity storable in the transmission buffer memory 3 will be exceeded. However, in such case, the feedback from the transmission buffer memory 3 causes the quantization step STEPG of the quantization circuit 16 to be increased in accordance with the increased remaining quantity data S25. Therefore, the quantity of the quantization data S16 corresponding to the deviation data S14 is reduced to thereby decrease the quantity of the transmission picture data S20. As a result, the overflow is prevented from taking place.
On the other hand, in the case of a drop in the remaining quantity data S25 down to an allowable lower limit, the feedback from the transmission buffer memory 3 controls the quantization step STEPG of the quantization circuit 16 to reduce the same in accordance with the remaining quantity data S25. In such case, the quantity of the transmission picture data S20 is incremented by increasing the generated quantity of the quantization data S16 corresponding to the deviation data S14. An underflow of the memory 3 is thus prevented.
It will be appreciated from the above that in the prior art picture data generating device 1 of FIGS. 2 and 4, the quantization step is controlled as a means for transmitting the significant picture information most efficiently while being adjusted to a transmitting condition under which the data transmitting velocity of the transmission data D.sub.TRANS is regulated on the basis of the transmission capacity of the transmission path 4. The foregoing follows from the emphasis placed on achieving a state in which the quantity of data remaining in the transmission buffer memory 3 invariably encounters neither an overflow nor an underflow. However, for picture data of certain types, the prior art arrangement may cause substantial deterioration of the picture quality corresponding to the transmitted picture data.
For example, in a picture PCX represented by the present frame data S12, as depicted in FIG. 5, the upper half picture data PCX1 is shown to have a relatively small amount of significant picture information, whereas, the lower half picture data PCX2 to be transmitted subsequent to the data PCX1 is shown to have an extremely large amount of significant picture information. In such case, when deviation data S14 corresponding to the upper half picture data PCX1 is quantized in the quantization circuit 16, the quantity of the data generated tends to decrease due to the small amount of significant picture information. Hence, the remaining quantity data S25 of the transmission buffer memory 3 decreases. In response thereto, the upper half picture data PCX1 is quantized with a much finer quantization step by changing the quantization step STEP G of the quantization circuit 16 to a smaller value. As a result, the data quantity of the transmission picture data S20 is incremented.
In contrast to the foregoing, when quantizing the deviation data S14 corresponding to the lower half picture data PCX2 subsequent to the data PCX1, the quantity of data generated from the lower half picture data PCX2 tends to increase. Therefore, the quantity of data S25 remaining in the transmission buffer memory 3 increases and, in response thereto, the quantization circuit 16 is controlled to increment the quantization step STEP G, thereby quantizing the lower half picture data PCX2 with a coarser or rougher quantization step. Thus, the quantity of the transmission picture data S20 is reduced.
However, if the foregoing procedure is followed, the picture quality or value of the quantized frame picture data corresponding to the lower half picture data PCX2 is deteriorated more conspicuously than that corresponding to the upper half picture data PCX1. This is likely to give rise to a disturbing impression when viewing the picture as a whole.
When recording the transmission data D.sub.TRANS transmitted through the transmission path 4, for example, by a CD recording device, there is a fixed data transmission quantity per frame transmissible to the transmission path 4. However, before quantizing the lower half picture data PCX2, a relatively large quantity of generated data is allocated to the upper half picture data PCX1 which contains a small amount of significant picture information. Hence, there is no choice but to transmit the lower half picture data PCX2 containing a large amount of significant picture information within the range of the remaining data generated quantity. It is therefore impossible to avoid substantial, readily apparent deterioration of the picture quality.
If the upper half picture data PCX1 is quantized with a relatively small quantization step, and if the quantization circuit 16 quantizes the lower half picture data PCX2 similarly with the small quantization step, the quantity of data supplied to the transmission buffer memory 3 as the transmission picture data S20 sharply increases because the data PCX2 has a large quantity of significant picture information. This leads to an overflow of the transmission buffer memory 3. On the other hand, if the prior art video signal coding circuit 2 of FIG. 4 is employed to prevent such overflow, the upper half picture data PCX1 containing a small quantity of significant information is quantized with a relatively small quantization step, thereby making it possible to transmit the data of a high quality picture, whereas, in the transmission of the lower half picture data PCX2 containing a large quantity of significant information, the picture data is roughly quantized by incrementing the quantization step and thereby reducing the quantity of data to be transmitted for avoiding overflow of the buffer memory. However, as earlier noted, this results in extreme deterioration of the quality of the transmissible picture data.
The video signal coding process employed in the above-described prior art system of FIGS. 2 and 4 is further insufficient for obtaining the transmission of data which presents a picture of high quality for the reason that such process is not adapted to reflect the nature of the picture to be coded.
Human spectral luminous efficacy is an important condition to be considered when estimating the quality of picture transmission. Unless this condition is considered, it is practically impossible to transmit a high quality picture.
One property involved in human spectral luminous efficacy is the so-called "visual masking effect". The .differential.visual masking effect" refers to the phenomenon observed when a complicated picture (containing a large amount of high frequency components) and a simple picture (containing a small amount of high frequency components) are quantized with the same quantization step, and it is more difficult to detect deterioration of the quality of the complicated picture than o the simple picture.
Hence, when a complicated picture is relatively roughly quantized by a large quantization step, the deterioration of picture quality is visually undetectable.
Another property of spectral luminous efficacy is embodied in Weber's law. According to Weber's law, when a stimulus B acts on the human visual sense and the stimulus B is changed by .DELTA. B, the least threshold .DELTA. B/B for sensing the change .DELTA. B is expressed as follows: ##EQU1##
In other words, Weber's law expresses the phenomenon that the least threshold is constant.
When this phenomenon is applied to the quantization of a differential signal of the picture, the value of the differential signal to be quantized becomes larger with an increasing error .DELTA. B thereof, which implies that it is difficult to detect the error. However, the video signal coding process according to the prior art does not take into account the visual properties associated with a picture to be quantized, and hence cannot realize the high picture quality that can be obtained from consideration of such properties.
When static and dynamic regions of a picture are intermixed with each other, picture information changes abruptly, as in the case of a picture of an edge of a moving object, that is, at the boundary between the static and dynamic regions. If such picture information is processed in accordance with the above described prior art so that the quantization step STEP G is controlled to cause the quantity of data remaining in the transmission buffer memory 3 to fall within a predetermined range, there is the danger that noises will be generated in the part of the picture, such as the edge of the moving part, where the picture information abruptly varies.
In this connection, the human spectral luminous efficacy for a motion picture is low in the dynamic region, that is, a region in which motion appears, of the picture information, whereas it is high in the static region where no motion appears. Hence, where the static and dynamic regions are mixed, it is possible to prevent deterioration of picture quality of the generated data, in the visual sense, even when the quantization step STEP G used for quantizing the dynamic region is incremented for enhancing the quantization efficiency.
However, if rough quantization is effected by incrementing the quantization step STEP G for the dynamic region, when quantizing a boundary of a picture part at which the picture information abruptly changes between the dynamic region and the static region, this results in the generation of noises at such boundary.
This phenomenon will probably also appear at the boundaries between dynamic regions exhibiting different motions.
When examining the content of a picture, it will be seen that, in the great majority of cases, the picture information abruptly varies, for example, as at an object edge, or at the boundary between a complicated picture region and a simple picture region. If such picture is roughly quantized by a quantization step of a large value, this results in so-called "mosquito noises" at the edge part or in the generation of transmission data which presents a picture in which the complicated picture region is not smoothly connected to the simple picture region.
Furthermore, in the picture data generating system 1 according to the prior art, the differential data S14 is discrete-cosine-transformed (DCT) in the transform coding circuit to obtain the transform coding data S15. In such case, the weight given to a low frequency component of a spatial frequency is increased, and weight given to a high frequency component thereof is decreased. As a result, the quantization step STEP G is incremented for the high frequency components as compared with the quantization step STEP G for the low frequency components of the spatial frequency By reason of this arrangement, there can be incremented weighting of a region where human spectral luminous efficacy is relatively high and deterioration is easily detected, whereas weighting of a region where the spectral luminous efficacy is low and the deterioration is hard to be detected can be decremented. Hence, compression efficiency of the picture data is improved while enhancing the subjective picture quality.
In fact, however, when variable weighting is employed without limits irrespective of the nature of the picture, the high frequency information may be compressed to cause fading of the picture, if the entire quantity of picture information is small and the picture contains a good deal of high frequency components of the spatial frequency. This results in deterioration of the picture quality. For instance, in the case where a part of the picture to be transmitted contains high frequency components but no low frequency components, if the high frequency information is compressed, there may be no signal remaining to be transmitted.